1997 ford thunderbird overheating 5.4 inline engine?

My car is overheating when i go up hill then when i level out it runs normal operating temperature. Things I have replaced.
-put a rebuild engine in
-new thermostat and gasket 165 degree
-new water pump
-flush coolant
-service transmission
-new belt tensioner and pulley
-new oem radiator fan (it is blowing the right way)
-new radiator
I'm not losing coolant. Have checked it for 2 weeks and have not added anything. There's no oil in coolant and no coolant in oil.

Try a 190 thermostat. Sounds crazy but you might be moving water through to fast to cool

Its to the point that i have to turn on my heater. If your wondering thats me up there asking the question

This doesn't seem unusual. Going up hill makes engine work harder. Harder work means more heat. If it's not actually overheating don't worry about.

First of all, Ford's 5.4L is a V8, not an inline. Second of all, the biggest engine Ford put in 1994-1997 T-Birds was a 4.6L, not a 5.4.

The first thing I would do is make sure there isn't trash or something in front of the radiator blocking airflow.

Then, check the coolant level. When you flushed it, air could have still been trapped in the system. You may just need a top off.

Next, replace that 160 degree thermostat with a 190. The 160 is staying open all the time and not letting the coolant stay in the radiator long enough to cool off.

Finally, if your rebuilt engine is a 5.4 and not the stock 4.6, your stock radiator might be too small. The stock 4.6L radiator is kinda dinky to start with. Dropping a bigger motor in the car isn't going to help. Get a bigger aftermarket radiator.